Had you asked Charles Darwin why people have sex, he would say that it’s all about procreation. You know, survival of the fittest and fastest breeders and all that.

Had you asked 1,500 students at the University of Texas in 2007 why they have sex, you’d have gotten 237 reasons (perhaps unsurprising, given the population) ranging from the blatantly self-centered (good exercise) to the altruistic (to please my partner).

But doesn’t the more relevant question have to do with the effect of those reasons on one’s sex life and relationship than about why a person has sex? After all, if you know that sex for your partner is simply an alternative to going to the gym, I suspect that would color your experience of sex and view of the relationship.

We know that more and better sex is linked to happiness and relationship satisfaction. But some of our motives for sex would seem to make our lives and relationships better and others to make them worse. And are there nuances within this paradigm—are some reasons for sex better than others, and is more sex always good? Finally, are there some practical applications for all this academic falderal?

Recently, in two separate studies, researchers at the University of Toronto quizzed a hundred or so dating and married couples about their reasons for having sex. The couples kept diaries for several weeks, answering questions whenever they had sex about their motivation, levels of desire, and how they felt about the relationship.

What the researchers found was that why we have sex on any given day does indeed affect how we feel about our relationship, our partner, and our level of desire. The effects were the same for both men and women, and they persisted for months after.

Responses were grouped into two main categories: approach, which seeks a positive outcome (I want to be closer to my partner) and avoidance, which seeks to circumvent something negative (I don’t want to feel guilty.) Motives for sex can also focus on oneself (I want to feel good) or one’s partner (I want to make my partner feel good.)

Researchers found that when respondents engaged in sex for partner-focused, approach motives, they felt more satisfied with the sex and better about their partner than those who had sex for self-focused, avoidance reasons.

The surprising element was that, when a person had sex for positive, partner-focused reasons, the partner also felt more positively about sex and the relationship, and that these effects persisted over time.

“If I am having sex more for approach goals, it increases my desire and satisfaction, so my partner probably senses that and it contributes to their outcome. Our satisfaction carries over to them.” says Dr. Amy Muise, lead researcher, in this article.

So it would seem that, while more sex is good, more sex for the right reasons is even better.

Of course, everyone has sex for a variety of reasons, depending on the day. Sometimes they’re positive and partner-focused (to give pleasure) and sometimes they’re negative and self-focused (to avoid guilt or conflict). And of course, we have sex when we aren’t particularly in the mood. But simply understanding the power and cumulative effect of positive, partner-focused motivation might encourage us to work on our attitude the next time our partner gets that look.

We might also work on the kind of communication and mutual respect that will make it easier for both partners to have sex for positive reasons more often. “Perhaps younger men and women still give in for this (avoidance) reasons,” says Iona Monk, counselor and founder of Vancouver Couples Counseling, in this article, “but I’d like to think it shifts as we mature, and learn to communicate better and know and accept our needs more.”